High frequency (HF) plasma process excitation configurations include an HF generator that supplies HF power to a plasma process (which is the plasma load). The HF power is normally supplied to the plasma process in a narrow frequency band, for example, between about 10-30 MHz, or at about industrial frequencies of 13.56 MHz or 27.12 MHz. A measuring device can be provided for measuring the power supplied to the plasma process. The supplied power is detected with the use of the measuring device in order to enable accurate closed-loop and/or open-loop control of the power.
There are different methods to measure or determine the power supplied to the plasma load of an HF plasma process excitation system. For example, the measuring device can be a directional coupler. A directional coupler can determine the forward power Pi (that is, the power supplied to the plasma load) and the reflected power Pr (that is, the power reflected from the plasma load). Directional couplers decouple part of the power that is guided through a transmission line (that is, a conductor that transmits electrical power of the HF generator to the plasma load) of the directional coupler.
If the transmission line is a strip line, then a directional coupler can be used to measure or determine the power. In this case, the directional coupler can have a length of one quarter of a wavelength λ that is inversely related to the frequency of the HF signal from the HF generator. Such lengths would be large for a system that is operated in a range of between 10 and 30 MHz.
The directional couplers disclosed in prior art, e.g., in DE 10 2004 021 535 A1 and in U.S. Publication No. 2005/0212617 A1 are designed for frequencies of, for example, 1 GHz and more, in which case strip line theory plays a substantial role.
In conventional directional, couplers, one coupling line extends parallel to the transmission line. A small part of the power is decoupled from the transmission line, which supplies the power of the generator to the load, to the coupling line through electrical and magnetic coupling. A power that is proportional to the reflected power can be tapped at one end of the coupling line, and a power that is proportional to the forward power can be tapped at the other end.